I was playing on Windows.
The problem with the present approach is, it is not mentioned when the back-up save is being triggered. Therefore, as long as the most current savefile is not reliably stored, poor players like myself will have to keep re-grinding (and this game is already quite grindy). If your aim is to prevent gamers from savefile cheats, please also consider the overall stability and weigh the relative game design importance.
This game has potential, but it also needs to be save-able. I suggest either 1) Create a separate hardcore mode which does not allow discretionary saving, or; 2) Autosave at the end of a battle/before the start of a battle + roll over the previous save to the back-up slots